OMGoodness, I just played my friend who mains MK today. At first, I lost around 3 rounds, then, I started playing calm and I won about 20-40 rounds. He won 2 times after that when I was sandbagging. I feel so good.
The Tornado is really tough to beat. A good MK can start heading from Tornado into Tornado if he starts to catch you in it.
I spammed the forward smash a lot. You have to space really well. What I did was, he'd often go for the down-smash to knock me up and then juggle me. I just stayed patient, rolled and spot-dodged at every appropriate moment. When he's Uair'ing you, DI to either side and spam the air dodge.
If he's below you and you know he has yet to attack you, I'd suggest pre-emptively air-dodging because he might use his B-UP attack. Watch out for the glide attack that follows the B-UP attack.
When he stayed on the ledge, I hung back and spammed arrows until he came off. I couldn't get near because I was afraid of his glide attack. Often, when he did his Tornado or Drill attack, I'd cancel it a ton of times with arrow accuracy (I LEARNED HOW TO LOOP ARROWS, AND I CAN'T STOP!). After I learned how to loop one arrow, I got carried away and would stop paying attention to beating on my opponent, prolly cause I was sandbagging, and just totally zone out and start looping arrows, lol.
I threw in some jabs to interrupt any combos he might start (Jabs are your best friend. Falco in Melee's jabs were awesome combo-interrupters or continuers). When he was rolling, I'd time a down-smash to hit him on either side of me when he was vulnerable during the roll.
To recover, I generally did a glide, dove then came at him at a horizontal for a glide attack. If you can time it right, you will almost never lose the priority-wrestle. (There would prolly not even be one.) What sucked was when I absolutely HAD to use my B-UP to reach the ledge where MK was waiting.
Two things, if he rolls at the same time you do it, you're not in a good position. Get ready to shield. Secondly, MK can nab you with a smash through the Angel Ring. A dash attack can surprise after your B-forward ends if your opponent is waiting for it to end.
You do not have to go them. (That's why I do not like DEHF's fighting style and love lmnz.) Spam the arrows (it'll stop a tornado but drills are arrow immune =$). Spot dodge, roll, walk back and forth (equivalent of wavedashing in Melee) then, when you know you can get it, punish! I hit him with my forward smash when he thought he was out of range. Walking back and forth can cause your opponent to make a feeble attempt at an aerial that will hit only air as you walk back in to punish.
Another specific thing about Pit vs MK. Often, my friend would go into a B-UP glide from the ledge. After defending with arrows, he'd go in for a glide attack. My trick to beating his was to roll toward him and he couldn't dive deep enough. I also counteracted with a forward smash but glide cancelling can match this.
I've also found that the glide attack is an AWESOME KO move! I love it SO SO SO MUCH! It can be unexpected when your opponent awaits your move. It kills people by sending them straight up and it's a nice way to finish your glide if you don't want to pull up.
Hmm, what else? He switched characters several times. Every time, I beat him because it was his metagame that I was watching. It's always, run in with an attack, I'd dodge it and punish (with a forward smash). He did pretty good with Dedede though. It was a bit easier to metagame D3 because he was so slow. My arrows could often outspam his B-forward. You can usually nail D3 with your Uair. A basic combo is down-throw to short-hopped Uair or Up-smash.
When MK down-throws, I air-dodge and jump behind him. Air-dodge to counter MK's air-game and Shuttle Loop. My friend often used a short or long A-combo and then chaining it to a down-smash. In this case, I could often either spam forward-smash as I DI'd out. (Hopefully you don't Smash DI back in.) Unless I knew that he'd probably connect with the following down-smash.
An option is to shield-grab if he comes in from the air, you don't want to dodge or think that you won't be able to get back at him with an Up-smash or Uair. Shielding your opponent's aerial, jump-cancel your shield into a short-hopped Dair. Short-hopped Dairs when the MK rushes in for an attack. You'll DI slightly left then back right to connect with the Dair while his attack hits the air beneath you. Up-smashes aren't good for neutralising an incoming MK aerial. I think he simply got hit by the first (few) strikes then settled beside me and connected with a down-smash.
When hanging on to the ledge with the opponent edgeguarding, options are: let go, multiple jumping over him or using a Fair. Get-up attack. How dare he think he can beat your get-up attack. Those things are a menace in Brawl. Let go, jump, shoot arrows.
Truly, the arrows are a menace.